The T cell receptor (TCR) is a multisubunit complex which functions as an antigen recognition cell surface structure and a signal transduction unit on T lineage cells. The TCR consists of the heterodimeric alpha and beta subunits associated with the monomorphic gamma, delta, epsilon, zeta, and eta subunits collectively known as CD3. All subunits with the exception of eta have been cloned at cDNA and genomic levels. Until now the primary structure of eta has not been defined but recent evidence implicates an important function for this protein in TCR signalling resulting in phosphoinositide turnover as well as in the cell cycle arrest and death of T-T hybridomas upon TCR crosslinking. We have recently deduced the primary structure of the murine CD3eta from protein microsequencing and cDNA cloning. The overall goal of this project is to understand the functional role of CD3eta in T cell receptor signalling and programmed cell death, both of thymocytes and T-T hybridomas. The specific aims of these proposed studies include: 1) the characterization of CD3eta genomic organization; 2) the evaluation of CD3eta expression during thymic development using mRNA analysis of thymocytes at different stages of development and tissue section staining with specific anti-CD3eta antibodies raised against peptides synthesized from deduced CD3eta protein sequence; 3) the reconstitution of murine CD3zeta and eta in mutant cell lines in order to elucidate their respective roles in signal transduction; and 4) site directed mutagenesis of CD3zeta and eta in order to understand the structure- function basis of the differences between CD3zeta and eta in signal transduction. These studies will elucidate the role of CD3eta protein in signal transduction and may lead to an understanding of the process of cell death which results in the selection of thymocytes during development.